1. Field of the Invention
The invention relates to a receiver in which a normal component and a quadrature component of a signal that is modulated on a carrier signal are directly generated by sampling the carrier signal, the sampling instant being determined by a sequence of sampling instants.
2. Description of the Related Art
In the article entitled "Quadrature Sampling of FM-Bandpass Signals--Implementation and Error Analysis", W. Rosenkranz, Proc. of the Int. Conf. on Digital Signal Processing, Florence, Italy, 1987, pp. 377 to 381, the sampling is described of frequency-modulated bandpass signals in which the real and imaginary parts of the complex envelope, the so-called quadrature components, are directly produced by sampling the modulated bandpass signals at a carrier frequency f.sub.c. For this purpose, an analog-to-digital converter is controlled by means of periodically recurrent sampling pulses having a frequency f.sub.Q, to be denoted as clock frequency f.sub.Q hereinafter.
The clock frequency f.sub.q is to be selected in such a way that a sampling interval T.sub.a for sampling the normal component, i.e. the real part of the bandpass signal, exactly corresponds to a half period of the carrier frequency or an integer multiple of this time interval respectively. With odd multiples, however, the sign of each second sample value can be inverted. In order to produce sample values of the quadrature component which is in quadrature to the sample values of the normal component thus produced, the input signal is to be sampled at instants which with respect to the sampling instants of the normal component present a time-delay of a quarter of the carrier signal period or a quarter plus an integer multiple of the half carrier signal period. Also in this case with odd multiples the sign of each second sample value has to be inverted. In this manner value pairs will be obtained corresponding to a sampling frequency of f.sub.a= 1/T.sub.a which correspond to the real and imaginary parts of the complex envelope.
In a circuit arrangement described in the article mentioned hereinbefore only a single analog-to-digital converter is necessary. The clock frequency f.sub.Q is selected in such a way that the clock interval T.sub.q= 1/f.sub.q is equal to a quarter of the period plus an integer multiple of the half period of the carrier frequency. Consequently, the two conditions mentioned hereinbefore are alternately fulfilled so that each sample Value alternately corresponds to the normal component and the quadrature component. By means of an interpolation filter the time-delay of I and Q components obtained in pairs is eliminated so that values corresponding to those of a simultaneous sampling of I and Q components are obtained at the output of the filter. Thus, the ratio of the carrier frequency f.sub.c to the half sampling frequency f.sub.a/ 2 is an integer and cannot be chosen arbitrarily.
A further condition is that the sampling frequency f.sub.a is at least equal to the bandwidth B of the bandpass signal to be sampled.
With bandpass signals whose signal of which the content is derived from a sequence of binary values based on a bit clock frequency f.sub.b, it is advantageous for the sampling frequency f.sub.a to be chosen to be equal to the bit clock frequency f.sub.b.
In transmission systems in which, for example, this bit clock frequency f.sub.b is a predetermined value, the choice of the carrier frequency f.sub.c in priorart arrangements is thus considerably restricted by the described items.